Conventionally, a full-wave voltage doubler circuit has been employed to boost an input voltage from 100V to 200V.
FIG. 16 is a diagram illustrating an example of a conventional full-wave voltage doubler circuit.
The full-wave voltage doubler circuit 10 comprises a bridge diode circuit 4 for rectifying an output voltage of an AC power supply 1, a power-factor improvement reactor 3 which is connected in series between the AC power supply 1 and the bridge diode circuit 4, two electrolytic capacitors 5 and 6 which are connected in series to each other and in parallel to the bridge diode circuit 4, and an electrolytic capacitor 9 which is connected in parallel to the electrolytic capacitors 5 and 6.
With reference to FIG. 16, input terminals 1a and 1b of the full-wave voltage doubler circuit are connected to an output terminal of the AC power supply 1. The bridge diode circuit 4 comprises two diodes 4a and 4b which are connected in series between output terminals 1c and 1d of the full-wave voltage doubler circuit 10, and a connection node 4c of the diodes 4a and 4b is connected to the input terminal 1a of the full-wave voltage doubler circuit 10 through the power-factor improvement reactor 3. Further, the other input terminal 1b of the full-wave voltage doubler circuit 10 is connected to a connection node of the electrolytic capacitors 5 and 6, and protection diodes 7 and 8 are connected in parallel to the electrolytic capacitors 5 and 6, respectively.
In the full-wave voltage doubler circuit 10 thus constituted, the output voltage of the AC power supply 1 is full-wave-rectified by the diodes 4a and 4b which are components of the bridge diode circuit 4, and the electrolytic capacitors 5 and 6 are alternately charged by a full-wave-rectified output from the bridge diode circuit 4 at a cycle equal to the cycle of the output voltage of the AC power supply 1. A voltage twice as high as the output voltage of the AC power supply 1, which is caused by this charging at both ends of the capacitors 5 and 6 connected in series, is smoothed by the electrolytic capacitor 9, and a smoothed double-high voltage is generated between the output terminals 1c and 1d of the full-wave voltage doubler circuit 10.
On the other hand, there has also been proposed another example of a full-wave voltage doubler circuit wherein, in order to increase efficiency, a metallized film capacitor is used as a voltage doubler capacitor to be charged by a rectified output of diodes connected in series, and two bridge diode circuits are connected in parallel (for example, refer to Japanese Published Patent Application No. 2001-211651 (FIG. 1)).
Further, there has also been proposed a circuit system wherein a rectifier circuit is provided with a booster circuit, in order to increase the power factor of an input power supply and boost the input voltage to an arbitrary voltage (for example, refer to Japanese Patent No. 3308993 (FIG. 1)).
FIG. 17 is a diagram for explaining a voltage conversion circuit disclosed in Japanese Patent No. 3308993.
The voltage conversion circuit 11 comprises a rectifier circuit 20 for rectifying an output voltage of an AC power supply 1 which is applied to input terminals 2a and 2b, a booster circuit 13 for boosting an output voltage of the rectifier circuit 20, and an electrolytic capacitor 17 which is charged by an output voltage of the booster circuit 13.
The rectifier circuit 20 comprises first and second diodes 21 and 22 which are connected in series, and third and fourth diodes 23 and 24 which are connected in series. A connection node 20a of the first and second diodes 21 and 22 is connected to an input terminal 2a of the voltage conversion circuit 11, and a connection node 20b of the third and fourth diodes 23 and 24 is connected to the other input terminal 2b of the voltage conversion circuit 11. Further, the cathodes of the first and third diodes 21 and 23 are connected to each other, and the connection node of the first and third diodes 21 and 23 is an output terminal of the rectifier circuit 20. The anodes of the second and fourth diodes 22 and 24 are connected to each other, and the connection node of the second and fourth diodes 22 and 24 is the other output terminal of the rectifier circuit 20.
The booster circuit 13 comprises a reactor 14 having an end connected to the other end of the rectifier circuit 20, a diode 16a having an anode connected to the other end of the reactor 14 and a cathode connected to the output terminal 2c of the voltage conversion circuit 11, and a switching element 15 which is connected between the connection node of the reactor 14 and the diode 16a and the other output terminal of the rectifier circuit 20. The switching element 15 is an IGBT (Insulated Gate type Bipolar Transistor), and a diode 16b is connected in inverse-parallel to the IGBT 15.
In the voltage conversion circuit 11, the AC voltage supplied from the AC power supply 1 is rectified by the rectifier circuit 20, and the output of the rectifier circuit 20 is input to the booster circuit 13. In the booster circuit 13, the output of the rectifier circuit 20 is boosted by on-off of the switching element 15. That is, an electric path at the output side of the reactor 14 is short-circuited when the switching element 15 is turned on, whereby a DC current flows from the rectifier circuit 20 into the reactor 14, and energy is stored in the reactor 14. Thereafter, when the switching element 15 is turned off, an induced voltage is generated in the reactor 14, and the capacitor 17 is charged by a sum voltage of the induced voltage and the output voltage of the rectifier circuit 20, whereby a voltage higher than the output voltage of the rectifier circuit 20 is generated between the terminals of the capacitor 17.
In the voltage conversion circuit 11 having the booster circuit 13 of this type, the current supplied from the AC power supply 1 is controlled so as to have a sinusoidal waveform by adjusting the time ratio between the on period and the off period of the switching element 15, whereby the power factor is improved. Further, the magnitude (absolute value) of the input current is controlled by adjusting the time ratio, whereby the level of the output DC voltage can be controlled.
However, the conventional full-wave voltage doubler circuit 10 shown in FIG. 16 requires the large-capacitance voltage-doubler capacitors 5 and 6 and the reactor 3 for improving the power factor. Further, if the capacitance of the voltage-doubler capacitor is small, the capacitor does not operate as a voltage-doubler capacitor.
In brief, the operation of the voltage-doubler circuit is as follows. That is, the two capacitors connected in series are alternately charged at every half period of the input AC voltage, and a sum voltage of the terminal voltages of the two capacitors is outputted. Therefore, when the capacitances of the capacitors are small, the terminal voltages of the charged capacitors are undesirably lowered during the half period of the input voltage when no charging is carried out, and the output voltage of the voltage-doubler circuit 10, which is output as a sum voltage of the terminal voltages of the two capacitors, is not double the input voltage.
On the other hand, the conventional voltage conversion circuit 11 shown in FIG. 17 is a component of, for example, a motor driving apparatus, and the capacitance of the reactor 14 as a component of the booster circuit 13 and the capacitance of the capacitor 17 charged by the output of the booster circuit 13 are determined according to the switching frequency of the switching element 15. That is, in order to reduce the capacitance of the reactor 14, the switching frequency must be increased so as to reduce the harmonic current that appears at the input end. Further, since the ripple of the voltage charged in the capacitor 17 is increased as the capacitance of the capacitor 17 is reduced, the switching frequency must be increased to reduce the ripple.
However, considering the efficiency of the voltage conversion circuit 11 or the cost of the harmonic switching element, there is a limitation in actually increasing the switching frequency by the booster circuit 13, and therefore, the capacitances of the reactor 14 and the capacitor 17 cannot be reduced by a predetermined value or more.
As described above, in the circuit structures such as the conventional full-wave voltage doubler circuit 10 and the voltage conversion circuit 11, since the capacitances of the capacitors and the reactors, which are components of these circuits, cannot be reduced by a predetermined value or more, the circuit scale of the full-wave voltage doubler circuit 10 or the voltage conversion circuit 11 cannot be reduced. Therefore, it is difficult to reduce the size of a motor driving apparatus including these circuits.